Linear chirped pulses in a Raman excitation scheme give a possibility to selectively excite only one target qubit in a quantum register without disturbing its neighbors. Selectivity is guaranteed by adiabaticity of the pulse excitation which allows manipulation by only resonant qubit and leaving all other qubits unperturbed.
Intense femtosecond pulses are used to carry out the proposed scheme. The large band-width of the pulse provides the necessary flexibility to manipulate a qubit by frequency components and to apply a time-dependent phase on the pulse. The high intensity of the femtosecond laser pulses provides enough power to maintain the high Rabi frequencies necessary for high-speed quantum gates.
Many approaches to forming quantum computers require selectively addressing one particular qubit in a quantum register without disturbing its neighbors. Strong focusing such that the laser beam does not disturb the neighboring qubits has to be applied to overcome this problem using laser radiation. In other words, the spatial extent of the beam has to be less than the distance between neighboring qubits. Another approach uses the frequency-selective excitation of the quantum register qubits, which can be achieved by making transition frequencies of the qubits sufficiently different. This can be done, for example, by using a “gradient trap” such as by applying gradient magnetic field to a spin-based quantum register or by applying a gradient electric field to properly trapped polar molecules or ions.
The requirement of selectively exciting only a single target qubit in a quantum register using transition frequency differences between neighboring qubits removes focusing difficulties. However, this restricts the intensity of the external fields. The Rabi frequency of the corresponding transition has to be much smaller than the difference in the transition frequencies between neighboring qubits. In turn, this requires a strong gradient in the external magnetic or electric field, which might be difficult to realize experimentally. Reducing the pulse intensity makes the Rabi frequency smaller; however, it slows gate operations. In short, both proposed methods place additional restrictions on the physical implementation of quantum gates, and make it difficult to find a suitable quantum system for a quantum register.